Why does knock retard kick in at exactly 4K RPM's???
 

Last Friday I helped dyno-tune a Z06 Vette w/ LT's & a 224 cam. The car was getting almost 10 degrees of knock retard, starting at exactly 4K RPM's. Using ATAP & LS1Edit, we were able to get the KR down to just a few degrees, picking up almost 20 HP. But we couldn't eliminate the KR entirely.

Then we dynoed my '99 SS. I previously was getting a lot of pinging & KR, limiting my total advance to 20 degrees. I spent a lot of time cleaning the carbon out of my engine, & was anxious to see if the KR would improve. It did --- but I am still getting 4 degrees of KR, starting exactly at 4K RPMs! It then tapers off, and by 6K RPM's it's almost all gone. We weren't set up to do LS1Edit on my car, so I wasn't able to play w/ the spark advance.

What happens at 4K RPMs to trigger the KR, and all at once? Is this false knock?